Abstract
A simple technique has been developed to compare the recovery of a variety of forepumps after exposure to helium. A counter-flow type configuration is employed with controlled helium mass flow providing a known gas load into the forepump under investigation. The forepump backs a turbomolecular pump above which the helium concentration is measured using a quadrupole mass spectrometer. The initial recovery (0–10 s) is governed by the speed and achievable ultimate characteristics of the forepump; after this period the residual helium measurement (attributable to retention in the pump) is used for comparison. The results of measurements of several oil-sealed rotary vane and scroll pumps from four manufacturers are presented. The variability in their performance spanned four orders of magnitude. Continuous inlet air bleeds and gas ballasting are shown to improve the long-term recovery.
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